Antiviral drug recognition and elevator-type transport motions of CNT3

Nucleoside analogs have broad clinical utility as antiviral drugs. Key to their systemic distribution and cellular entry are human nucleoside transporters. Here, we establish that the human concentrative nucleoside transporter 3 (CNT3) interacts with antiviral drugs used in the treatment of coronavirus infections. We report high-resolution single-particle cryo-electron microscopy structures of bovine CNT3 complexed with antiviral nucleosides N4-hydroxycytidine, PSI-6206, GS-441524 and ribavirin, all in inward-facing states. Notably, we found that the orally bioavailable antiviral molnupiravir arrests CNT3 in four distinct conformations, allowing us to capture cryo-electron microscopy structures of drug-loaded outward-facing and drug-loaded intermediate states. Our studies uncover the conformational trajectory of CNT3 during membrane transport of a nucleoside analog antiviral drug, yield new insights into the role of interactions between the transport and the scaffold domains in elevator-like domain movements during drug translocation, and provide insights into the design of nucleoside analog antiviral prodrugs with improved oral bioavailability.

Structural insights into the FtsEX-EnvC complex regulation on septal peptidoglycan hydrolysis in Vibrio cholerae

During bacterial cell division, hydrolysis of septal peptidoglycan (sPG) is crucial for cell separation. This sPG hydrolysis is performed by the enzyme amidases whose activity is regulated by the integral membrane protein complex FtsEX-EnvC. FtsEX is an ATP-binding cassette transporter, and EnvC is a long coiled-coil protein that interacts with and activates the amidases. The molecular mechanism by which the FtsEX-EnvC complex activates amidases remains largely unclear. We present the cryo-electron microscopy structure of the FtsEX-EnvC complex from the pathogenic bacteria V. cholerae (FtsEX-EnvCVC). FtsEX-EnvCVC in the presence of ADP adopts a distinct conformation where EnvC is "horizontally extended" rather than "vertically extended". Subsequent structural studies suggest that EnvC can swing between these conformations in space in a nucleotide-dependent manner. Our structural analysis and functional studies suggest that FtsEX-EnvCVC employs spatial control of EnvC for amidase activation, providing mechanistic insights into the FtsEX-EnvC regulation on septal peptidoglycan hydrolysis.

Molecular basis of polyspecific drug and xenobiotic recognition by OCT1 and OCT2

A wide range of endogenous and xenobiotic organic ions require facilitated transport systems to cross the plasma membrane for their disposition. In mammals, organic cation transporter (OCT) subtypes 1 and 2 (OCT1 and OCT2, also known as SLC22A1 and SLC22A2, respectively) are polyspecific transporters responsible for the uptake and clearance of structurally diverse cationic compounds in the liver and kidneys, respectively. Notably, it is well established that human OCT1 and OCT2 play central roles in the pharmacokinetics and drug-drug interactions of many prescription medications, including metformin. Despite their importance, the basis of polyspecific cationic drug recognition and the alternating access mechanism for OCTs have remained a mystery. Here we present four cryo-electron microscopy structures of apo, substrate-bound and drug-bound OCT1 and OCT2 consensus variants, in outward-facing and outward-occluded states. Together with functional experiments, in silico docking and molecular dynamics simulations, these structures uncover general principles of organic cation recognition by OCTs and provide insights into extracellular gate occlusion. Our findings set the stage for a comprehensive structure-based understanding of OCT-mediated drug-drug interactions, which will prove critical in the preclinical evaluation of emerging therapeutics.

Molecular basis of polyspecific drug binding and transport by OCT1 and OCT2

A wide range of endogenous and xenobiotic organic ions require facilitated transport systems to cross the plasma membrane for their disposition 1, 2 . In mammals, organic cation transporter subtypes 1 and 2 (OCT1 and OCT2, also known as SLC22A1 and SLC22A2, respectively) are polyspecific transporters responsible for the uptake and clearance of structurally diverse cationic compounds in the liver and kidneys, respectively 3, 4 . Notably, it is well established that human OCT1 and OCT2 play central roles in the pharmacokinetics, pharmacodynamics, and drug-drug interactions (DDI) of many prescription medications, including metformin 5, 6 . Despite their importance, the basis of polyspecific cationic drug recognition and the alternating access mechanism for OCTs have remained a mystery. Here, we present four cryo-EM structures of apo, substrate-bound, and drug-bound OCT1 and OCT2 in outward-facing and outward-occluded states. Together with functional experiments, in silico docking, and molecular dynamics simulations, these structures uncover general principles of organic cation recognition by OCTs and illuminate unexpected features of the OCT alternating access mechanism. Our findings set the stage for a comprehensive structure-based understanding of OCT-mediated DDI, which will prove critical in the preclinical evaluation of emerging therapeutics.

Using the inner membrane of Escherichia coli as a scaffold to anchor enzymes for metabolic flux enhancement

Clustering enzymes in the same metabolic pathway is a natural strategy to enhance productivity. Synthetic protein, RNA and DNA scaffolds have been designed to artificially cluster multiple enzymes in the cell, which require complex construction processes and possess limited slots for target enzymes. We utilized the Escherichia coli inner cell membrane as a native scaffold to cluster four fatty acid synthases (FAS) and achieved to improve the efficiency of fatty acid synthesis in vivo. The construction strategy is as simple as fusing target enzymes to the N-terminus or C-terminus of the membrane anchor protein (Lgt), and the number of anchored enzymes is not restricted. This novel device not only presents a similar efficiency in clustering multiple enzymes to that of other artificial scaffolds but also promotes the product secretion, driving the entire metabolic flux forward and further increasing the gross yield compared with that in a cytoplasmic scaffold system.

Methotrexate recognition by the human reduced folate carrier SLC19A1

Folates are essential nutrients with important roles as cofactors in one-carbon transfer reactions, being heavily utilized in the synthesis of nucleic acids and the metabolism of amino acids during cell division^1,2. Mammals lack de novo folate synthesis pathways and thus rely on folate uptake from the extracellular milieu³. The human reduced folate carrier (hRFC, also known as SLC19A1) is the major importer of folates into the cell^1,3, as well as chemotherapeutic agents such as methotrexate^4-6. As an anion exchanger, RFC couples the import of folates and antifolates to anion export across the cell membrane and it is a major determinant in methotrexate (antifolate) sensitivity, as genetic variants and its depletion result in drug resistance^4-8. Despite its importance, the molecular basis of substrate specificity by hRFC remains unclear. Here we present cryo-electron microscopy structures of hRFC in the apo state and captured in complex with methotrexate. Combined with molecular dynamics simulations and functional experiments, our study uncovers key determinants of hRFC transport selectivity among folates and antifolate drugs while shedding light on important features of anion recognition by hRFC.

Structural basis for inhibition and regulation of a chitin synthase from Candida albicans

Chitin is an essential component of the fungal cell wall. Chitin synthases (Chss) catalyze chitin formation and translocation across the membrane and are targets of antifungal agents, including nikkomycin Z and polyoxin D. Lack of structural insights into the action of these inhibitors on Chs has hampered their further development to the clinic. We present the cryo-EM structures of Chs2 from Candida albicans (CaChs2) in the apo, substrate-bound, nikkomycin Z-bound, and polyoxin D-bound states. CaChs2 adopts a unique domain-swapped dimer configuration where a conserved motif in the domain-swapped region controls enzyme activity. CaChs2 has a dual regulation mechanism where the chitin translocation tunnel is closed by the extracellular gate and plugged by a lipid molecule in the apo state to prevent non-specific leak. Analyses of substrate and inhibitor binding provide insights into the chemical logic of Chs inhibition, which can guide Chs-targeted antifungal development.

Vanilloid-dependent TRPV1 opening trajectory from cryoEM ensemble analysis

Single particle cryo-EM often yields multiple protein conformations within a single dataset, but experimentally deducing the temporal relationship of these conformers within a conformational trajectory is not trivial. Here, we use thermal titration methods and cryo-EM in an attempt to obtain temporal resolution of the conformational trajectory of the vanilloid receptor TRPV1 with resiniferatoxin (RTx) bound. Based on our cryo-EM ensemble analysis, RTx binding to TRPV1 appears to induce intracellular gate opening first, followed by selectivity filter dilation, then pore loop rearrangement to reach the final open state. This apparent conformational wave likely arises from the concerted, stepwise, additive structural changes of TRPV1 over many subdomains. Greater understanding of the RTx-mediated long-range allostery of TRPV1 could help further the therapeutic potential of RTx, which is a promising drug candidate for pain relief associated with advanced cancer or knee arthritis.

Dental age estimation of Tibetan children and adolescents: Comparison of Demirjian, Willems methods and a newly modified Demirjian method

Tibetan ethnic group is one of the oldest ethnic groups in China and South Asia. This study set out to analyze the dental development and validate Demirjian method and Willems method in estimating dental age of Tibetan children and adolescents, and to modify Demirjian method based on Tibetan population to provide ethnic-specific reference data and a more reliable method for forensic age assessment in Tibetan ethnic group. In this study, 1951 samples aged between 4 and 15 years were retrospectively collected and analyzed. Multiple linear regression was used to establish relationship between chronological age (CA) and developmental stages of left mandibular permanent teeth. The accuracy of the modified method was tested and compared with that of Demirjian and Willems method. Results showed that dental maturity score (DMS) was significantly greater in girls than in boys in all age groups except for the 4-year age group (p < 0.05). Mean absolute error (MAE) was 0.96 years for both boys and girls by Demirjian method, and 1.06 and 1.16 years for boys and girls respectively by Willems method. Adjusted scores table was established and tested. The age of boys was overestimated by 0.13 years and the age of girls was underestimated by 0.06 years by the adjusted scores table. MAE was lower than that of the other two methods. In conclusion, Demirjian method and Willems method was not sufficiently accurate in estimating dental age of Tibetan population. The modified method was more suitable for dental age estimation of Tibetan population than Demirjian and Willems method.

Sample preparation of the human TRPA1 ion channel for cryo-EM studies

The transient receptor potential ankyrin 1 (TRPA1) ion channel is a member of the TRP channel family that is involved in sensing noxious stimuli that elicit pain and inflammation. Because of its critical physiological role and therapeutic importance, great efforts have been made to understand the structure and mechanism of TRPA1. Several human TRPA1 structures have been reported using single particle cryo-electron microscopy (cryo-EM) over the last 6 years. Here, we present a protocol for the heterologous expression, large-scale purification, and nanodisc reconstitution of the human TRPA1 channel for cryo-EM and biochemical studies.

Structural Insights into Electrophile Irritant Sensing by the Human TRPA1 Channel

Transient receptor potential channel subfamily A member 1 (TRPA1) is a Ca2+-permeable cation channel that serves as one of the primary sensors of environmental irritants and noxious substances. Many TRPA1 agonists are electrophiles that are recognized by TRPA1 via covalent bond modifications of specific cysteine residues located in the cytoplasmic domains. However, a mechanistic understanding of electrophile sensing by TRPA1 has been limited due to a lack of high-resolution structural information. Here, we present the cryoelectron microscopy (cryo-EM) structures of nanodisc-reconstituted ligand-free TRPA1 and TRPA1 in complex with the covalent agonists JT010 and BITC at 2.8, 2.9, and 3.1 Å, respectively. Our structural and functional studies provide the molecular basis for electrophile recognition by the extraordinarily reactive C621 in TRPA1 and mechanistic insights into electrophile-dependent conformational changes in TRPA1. This work also provides a platform for future drug development targeting TRPA1.

P1890Pioglitazone inhibits diabetes-induced atrial mitochondrial oxidative stress and improves mitochondrial biogenesis, dynamics and function through the PGC-1 signaling pathway

Background

Oxidative stress contributes to adverse atrial remodeling in diabetes mellitus. This can be prevented by the PPAR-γ agonist pioglitazone through its anti-oxidant and anti-inflammatory effects.

Purpose

In this study, the molecular mechanisms underlying these effects were investigated.

Methods

Rabbits were randomly divided into control (C), diabetic (DM), and pioglitazone-treated DM (Pio) groups. Echocardiographic, hemodynamic, electrophysiological, intracellular Ca2+ properties were measured. Serum PPAR-γ levels, serum and tissue oxidative stress and inflammatory markers, mitochondrial morphology, reactive oxygen species (ROS) production rate, respiratory function, and mitochondrial membrane potential (MMP) levels were measured. Protein expression of pro-fibrotic marker transforming growth factor β1 (TGF-β1), and the mitochondrial proteins (PGC-1α, fission and fusion-related proteins) were measured.

Results

Compared with controls, the DM group demonstrated larger left atrial diameter and fibrosis area associated with a higher incidence of inducible AF. Lower serum PPAR-γ level was associated with lower PGC-1α, higher NF-κB and higher TGF-β1 expression. Mn-SOD protein was not different but lower mitochondrial fission- and fusion-related proteins were detected. Mitochondrial swelling, higher mitochondrial ROS, lower respiratory control rate, lower MMP and higher intracellular Ca2+ transients were observed. In the Pio group, reversal of structural remodeling and lower inducible AF incidence were associated with higher PPAR-γ and PGC-1α. NF-κB and TGF-β1 were lower and biogenesis, fission and fusion-related protein were higher. Mitochondrial structure and function, and intracellular Ca2+ transients were improved. In HL-1 cell line, transfected with PGC-1α siRNA blunted the effect of pioglitazone on Mn-SOD protein expression and MMP collapse in H2O2-treated cells.

Conclusion

Diabetes mellitus induces adverse atrial structural and electrophysiological remodeling, abnormal Ca2+ handling and mitochondrial damage and dysfunction. Pioglitazone prevented these abnormalities through the PPAR-γ/PGC-1α pathway.

Acknowledgement/Funding

National Natural Science Foundation of China (No 81570298, 81270245, 30900618 to T.L.)

Cryo-electron microscopy structure of the lysosomal calcium-permeable channel TRPML3

The modulation of ion channel activity by lipids is increasingly recognized as a fundamental component of cellular signalling. The transient receptor potential mucolipin (TRPML) channel family belongs to the TRP superfamily and is composed of three members: TRPML1-TRPML3. TRPMLs are the major Ca²⁺-permeable channels on late endosomes and lysosomes (LEL). They regulate the release of Ca²⁺ from organelles, which is important for various physiological processes, including organelle trafficking and fusion. Loss-of-function mutations in the MCOLN1 gene, which encodes TRPML1, cause the neurodegenerative lysosomal storage disorder mucolipidosis type IV, and a gain-of-function mutation (Ala419Pro) in TRPML3 gives rise to the varitint-waddler (Va) mouse phenotype. Notably, TRPML channels are activated by the low-abundance and LEL-enriched signalling lipid phosphatidylinositol-3,5-bisphosphate (PtdIns(3,5)P₂), whereas other phosphoinositides such as PtdIns(4,5)P₂, which is enriched in plasma membranes, inhibit TRPMLs. Conserved basic residues at the N terminus of the channel are important for activation by PtdIns(3,5)P₂ and inhibition by PtdIns(4,5)P₂. However, owing to a lack of structural information, the mechanism by which TRPML channels recognize PtdIns(3,5)P₂ and increase their Ca²⁺ conductance remains unclear. Here we present the cryo-electron microscopy (cryo-EM) structure of a full-length TRPML3 channel from the common marmoset (Callithrix jacchus) at an overall resolution of 2.9 Å. Our structure reveals not only the molecular basis of ion conduction but also the unique architecture of TRPMLs, wherein the voltage sensor-like domain is linked to the pore via a cytosolic domain that we term the mucolipin domain. Combined with functional studies, these data suggest that the mucolipin domain is responsible for PtdIns(3,5)P₂ binding and subsequent channel activation, and that it acts as a 'gating pulley' for lipid-dependent TRPML gating.

Crystallization and preliminary crystallographic analysis of defective pollen wall (DPW) protein from Oryza sativa

The defective pollen wall (dpw) gene of Oryza sativa encodes a fatty acid reductase (DPW) which plays important roles in primary fatty alcohol synthesis. DPW catalyzes the synthesis of 1-hexadecanol. The enzyme shows a higher specificity for palmitoyl-ACP than for palmitoyl-CoA as the substrate, and can only use NADPH as the cofactor. To gain an understanding of the molecular mechanism underlying the reaction catalyzed by DPW, the gene encoding DPW without the N-terminal 80 amino acids (DPWΔ80) was cloned into pET-28a vector and was overexpressed in Escherichia coli. DPWΔ80 was purified to homogeneity and screened for crystallization. DPWΔ80 in complex with NADPH produced crystals that diffracted X-rays to a resolution of 3.4 Å. The crystals belonged to space group P6₁ or P6₅, with unit-cell parameters a=b=222.8, c=114.0 Å, α=β=90, γ=120°.

Regulation of nucleocytoplasmic localizatioin of the atDjC6 chaperone protein

The sequence of the atDjC6 chaperone protein includes three potential nuclear localization signal (NLS) sequences (A-C) and three potential nuclear export signal (NES) sequences (X-Z). The subcellular localization of atDjC6 was studied by scanning laser confocal microscopy of chimera with the green-fluorescent protein (GFP) transiently expressed in tobacco BY-2 cells. The localization of the atDjC6::GFP chimera was coincident with that of the nuclear stain propidium iodide. Site-directed mutagenesis was used to verify the predicted NLS sequences. Each was individually fused to GFP and tested for protein localization. The individual NLS sequences were sufficient to direct partial nuclear localization of GFP, although the targeting information within NLS-B is apparently conformation sensitive. Site-directed mutagenesis of the NES sequences increased the amount of each chimera that was nuclearly localized, indicating a decrease in nuclear export. When any pair of NLS sequences were appended to GFP, the chimera were entirely nuclearly localized. Quantitative two-hybrid analysis was used to verify that the decoding of NLS sequence information involves interaction with the NLS-receptor protein importin-alpha. Each of the NLS sequences is flanked by a site of potential Ser phosphorylation, and recombinant atDjC6 could be phosphorylated in vitro. Mutagenesis of Ser residues to the P-Ser mimic Asp interfered with nuclear targeting, apparently by preventing recognition or binding by importin-alpha. Our results are consistent with a regulated nucleocytoplasmic localization of the atDjC6 chaperone protein.

[Inhaled budesonide for severe asthma at high altitude]

OBJECTIVE

To determine the benefit of inhaled high dose budesonide combined with terbutaline in patients with severe asthma at high altitude.

METHODS

42 patients with severe asthma at high altitude were assigned in a randomized, double-blind fashion to receive either budesonide combined with terbutaline (budesonide group, 21 cases) or terbutaline with placebo (control group, 21 cases). Both groups received terbutaline delivered by a metered-dose inhaler in a dose of 2.5 mg. Budesonide delivered by a metered-dose inhaler in 1.2 mg in budesonide group and placebo managed by a specially prepared metered-dose inhaler in control group were administered after terbutaline treatment above procedures. Repeated once after 10 min. Before and after therapy, the scores of the activity of accessory respiratory muscles, dyspnea, wheezing (clinical index) and lung function were documented.

RESULTS

At 1, 2, 4, 6 h after therapy, FEV1% [(43 +/- +/- 5)%, (50 +/- 5)%, (57 +/- 5)%, (67 +/- 6)%], PEF% [(47 +/- 5)%, (55 +/- 6)%, (62 +/- 7)%, (69 +/- 7)%], clinical index (5.1 +/- 0.8, 4.3 +/- 0.6, 3.5 +/- 0.6, 2.5 +/- 0.4) in budesonide group and FEV1% [(42 +/- 5)%, (44 +/- 5)%, (45 +/- 5)%, (45 +/- 5)%], PFE% [(46 +/- 5)%, (47 +/- 5)%, (49 +/- 6)%, (49 +/- 6)%], clinical index (5.3 +/- 0.7, 5.0 +/- 0.5, 4.9 +/- 0.7, 4.8 +/- 0.7) in control group were difference markedly compared with before therapy [budesonide group was (35 +/- 5)%, (38 +/- 5)%, 8.3 +/- 1.0, and control group was (33 +/- 5)%, (38 +/- 5)%, 8.3 +/- 1.1, respectively], all (P < 0.01).

CONCLUSIONS

High dose of inhaled budesonide combined with terbutaline should be an effective therapy for patients with severe asthma at high altitude. Budesonide inhaled in high dose produces therapeutic effects as soon as 2 h after therapy.

[A fluorometric method for the determination of norepinephrine in tissue]

This paper reported the determination of norepinephrine (NE) by spectrofluorometry in tissue. The linear relationship between the flecoroscence intensity and the content of NE was in the range of 10-100 ng (lambdaex/lambdaem = 410 nm/505 nm). The rate of recovery is 91.68%-96.64% and RSD (n = 10) is 6.1%. This method is simple, rapid and sensitive.

[Spectrofluorometric determination of corticosterone in plasma and tissue]

Corticosterone is one of the adrenal hormones. This paper reported the determination of corticosterone in plasma and tissue of small mammals with spectrofluorometry. The linear relationship between the fluorescence intensity and the concentration of corticosterone is over the range of 0.01-0.24 microg/mL (lambdaex = 470nm, lambdaem = 525nm). The average rate of standard recovery is 96.0% and RSD (n = 10) is 4.2%. This method is simple, rapid and sensitive.

[Hydride generation-AFS determination of trace arsenic, antimony, selenium and mercury in Chinese herbal medicine in Qinghaixizang plateau]

In this paper, a hydride generation non-dispersive atomic fluorescence spectrometry method has been developed for the determination of trace arsenic, antimony, selenium and mercury in Chinese herbal medicine in Qinghai-Xizang plateau. Under the optimum condition, the quantities of fourteen interference elements are below certain non-interferent limits. The detection limits were as follow (g/mL): As 1.3, Sb 0.39, Se 0.24 and Hg 0.44. The relative standard deviation of 9 determination was As 2.4-4.2%, Sb 2.5-5.6%, Se 3.1-5.0% and Hg 3.8-5.9%. The recovery of four elements was 94.7-105.4%. This method is simple, rapid, sensitive and accurate, and has been successfully tested with As, Sb, Se and Hg in more than 20 kinds of Chinese herbal medicine in Qinghai-Xizang plateau.

Transient expression of uidA constructs in in vitro onion (Allium cepa L.) cultures following particle bombardment and Agrobacterium-mediated DNA delivery

Particle bombardment and Agrobacterium-mediated DNA delivery into immature embryos and microbulbs were used to investigate the expression of the uidA gene in in vitro onion cultures. Both methods were successful in delivering DNA and subsequent uidA expression was observed. Optimal transient β-glucuronidase activity was observed in immature embryos that had been pre-cultured for three days and bombarded at a distance of 3 cm from the stopping plate, under 25 in Hg vacuum, using 900-1300 psi rupture discs. The CaMV35S-uidA gene construct gave five fold higher transient β-glucuronidase activity than the uidA gene construct regulated by any of four other promoters initially chosen for high experession in monocotyledonous tissues.

The energetic linkage of GTP hydrolysis and the synthesis of activated sulfate

ATP sulfurylase, from Escherichia coli K-12, catalyzes both the hydrolysis of GTP and the synthesis of activated sulfate (APS). This paper describes the energetic linkage of these reactions and the events that couple them. Steady-state and single-turnover experiments suggest that the binding of GTP inhibits APS production and that the hydrolysis of GTP is required to generate the enzyme form(s) that produces APS. It is this progression from the inhibitory, E-GTP, to the productive, E-GDP, complexes in the cycle of APS synthesis that energetically links these two reactions. This model stands in contrast to other GTPase/target systems in which the binding of GTP alone is sufficient to catalyze multiple turnovers of the target reaction. The stoichiometry of GTP hydrolysis to APS synthesis is 1:1, and equilibrium measurements show that -9.1 kcal/mol, produced by the hydrolysis of GTP, is used to thermodynamically drive production of APS and PPi. These findings establish the mechanism of energy transfer in this novel GTPase/target system, and substantially alter our understanding of the energetics of sulfate activation, an essential step in the metabolic assimilation of sulfur.

The DNA sequence of the sulfate activation locus from Escherichia coli K-12

The DNA sequence of the sulfate activation locus from Escherichia coli K-12 has been determined. The sequence includes the structural genes encoding the enzymes ATP sulfurylase (cysD and cysN) and APS kinase (cysC) which catalyze the synthesis of activated sulfate. These are the only genes known to reside in the sulfate activation operon. Consensus elements of the operon promoter were identified, and the start codons and open reading frames of the Cys polypeptides were determined. During this work, another gene, iap, was partially sequenced and mapped. The activity of ATP sulfurylase is stimulated by an intrinsic GTPase. Comparison of the primary sequences of CysN and Ef-Tu revealed that CysN has conserved many of the residues integral to the three-dimensional structure important for guanine nucleotide binding in Ef-Tu and RAS. nodP and nodQ, from Rhizobium meliloti, are essential for nodulation in leguminous plants. The Cys and Nod proteins are remarkably similar. NodP appears to be the smaller subunit of ATP sulfurylase. NodQ encodes homologues of both CysN and CysC; thus, these enzymes may be covalently associated in R. meliloti. The consensus GTP-binding sequences of NodQ and CysN are identical suggesting that NodQ encodes a regulatory GTPase.

GTPase-mediated activation of ATP sulfurylase

GTP stimulates the synthesis of APS (adenosine 5'-phosphosulfate) by the enzyme ATP sulfurylase (ATP:sulfate adenylyltransferase, EC 2.7.7.4) via a GTPase mechanism. The activation of the enzyme, purified from Escherichia coli, is titratable with GTP. The initial rate of APS formation is increased 116-fold at a saturating concentration of GTP. The enzyme exhibits a GTPase activity that is stimulated by ATP and further enhanced by SO4; however, SO4 alone does not significantly stimulate GTP hydrolysis. The larger subunit of ATP sulfurylase, encoded by cysN, contains a GTP-binding consensus sequence common to other known GTP-binding proteins. This is the first evidence that the sulfate activation pathway is a metabolic target for regulation by a GTPase.